A diverse range of techniques and methods occupy the field of remote sensing. These methods employ both passive and active systems to provide a representation of the scene or object of interest and can exploit acoustic as well as electromagnetic energy as a source of illumination. Some systems employ real apertures, in which the active sensor dimensions are limited to the physical extent of a single sensor. Other systems use synthetic apertures, which derive much larger effective sensor dimensions through either 1), motion of a single sensor across an extended distance or 2), the distribution of multiple, discrete sensor elements across an extended area.
Whereas the directionality of a single, stationary antenna or sensor is essentially fixed, interesting properties may be obtained using multiple send/receive elements, particularly when arranged in an array pattern. With particular regard to antenna technology, traditional methods of producing a movable beam usually involved physically rotating a directional element. With a fixed array of antenna elements, however, beam forming may be used to generate a highly directional signal without any moving parts. This may also allow the direction of radiation pattern to change quickly or even broadcast in several directions at once.
Conventional beam forming processes steer the equivalent main lobe of the sensor array to achieve directional orientation. Certain beam-forming processes exploit interference effects, but the phase delays of the individual sensor elements are arbitrarily controlled to force a preferred overall sensing angle and beam width. The returned energy level |B(t,θ,φ)|2 for a given sensing direction (θ,φ) is the primary data of interest. The field of view must be scanned to build up a spatial profile of the scene.